In phase change (also referred to as thermoplastic or hot melt) ink jet printers, the ink itself is typically a wax-like substance which is solid (frozen) at room temperature, becoming liquid when heated. Once in liquid form, a suitable ink-jetting mechanism of the drop-on-demand (also known as impulse) or continuous types--both of which are equally applicable to the present invention--is employed to deposit the ink a drop at a time on a surface (also referred to as the substrate or print medium), typically, but not necessarily, paper. [Hereafter, the term "paper" is used to include paper and all other print substrates or medium.] Upon contact with the paper surface the ink resolidifies.
In an impulse ink jet apparatus, a piezoelectric transducer (or like device) encircling a chamber of ink is selectively energized in response to the application of a voltage thereby causing the transducer to contract so as to jet out (expel) a droplet of ink through an orifice in the print head toward the substrate. The voltage is then removed to de-energize the transducer thereby causing it to expand and be refilled with ink from the chamber. Thus, the ink is ejected "on demand" in that the ink is ejected only when the transducer is energized. On the other hand, in continuous ink jet printers the ink droplets are continuously ejected at regular intervals and deflected away from the substrate where not desired. See, for example, U.S. Pat. Nos. 3,715,219 and 3,653,932 for a disclosure of a continuous jet with a phase change ink and such patents are hereby incorporated by reference.
For a discussion of phase change inks and the use thereof in impulse ink jet apparatuses suitable for application of the present invention, reference is made to the following patents and patent applications, all of which are assigned to the same assignee as the present invention and all of which are hereby incorporated herein by reference:
(1) Merrit U.S. Pat. No. 4,390,369 entitled "Natural Wax-Containing Ink Jet Inks" which issued June 28, 1983.
(2) Pending U.S. patent application Ser. No. 146,211 entitled "Hot Melt Ink Jet With Dispersing Solid Pigment In A Hot Melt Vehicle," filed on Jan. 1, 1988 which is a continuation of Ser. No. 006,727 filed on Jan. 23, 1987 (now abandoned), which is in turn a continuation of Ser. No. 668,095 filed on Nov. 5, 1984 (now abandoned).
(3) Howkins U.S. Pat. No. 4,459,601 entitled "Ink jet Method and Apparatus" which issued on July 10, 1984.
(4) Guiles U.S. Pat. No. 4,791,439 entitled "Ink Jet Apparatus With Improved Reservoir System For Handling Hot Melt Ink" which issued on Dec. 13, 1988.
(5) Pending U.S. patent application Ser. No. 093,151 entitled "A Demand Ink Jet Utilizing A Phase Change Ink And Method Of Operating," filed on Sept. 2, 1987 and currently on appeal which is a continuation of Ser. No. 938,334 filed on Dec. 4, 1986 (now abandoned), which is a continuation of Ser. No. 610,627 filed on May 16, 1984 (now abandoned).
Suitable ink jet print heads include those comprised of a stacked, slanted linear array of nozzles, typically 2.sup.n nozzles where n is a positive integer (e.g., 16, 32, 48 or 64 individual nozzles), each nozzle being driven by a separate piezoelectric powered piston pump or similar actuator. By way of example only, the following discussion is based on a 16-nozzle print head as shown in FIG. 1. Although not drawn to scale in FIG. 1, a suitable horizontal distance H between two adjacent nozzles (called the inter-jet spacing) is 14/240 inches, while a suitable vertical distance V between two adjacent nozzles is 1/240 inches.
The paper may be viewed as being comprised of a plurality of parallel horizontal rows--called pixel (picture element) rows-upon which dots (droplets) of ink may be placed. The vertical distance between the pixel rows is the same as the vertical distance V between the nozzles of the print head. Each of the 16 nozzles is responsible for the coverage of one pixel row per print pass. During a first print pass as the head is shuttled in a first direction R (left to right in FIG. 1) it is possible, by firing all nozzles throughout the pass, to print essentially 16 pixel rows of dots. The result is an essentially solid block of ink on the paper having a height of approximately 15*V.
The paper may then be stepped or advanced (typically through use of a stepper motor) vertically by 16 pixel rows (i.e., 16*V) after completion of each print pass so that the next 16 pixel rows of the paper may be printed on. [Since what is important is relative movement between the print head and the substrate, alternatively, the print head can be moved or the print head and substrate can both be moved.] Then, during a second print pass as the head is shuttled in a second direction L (right to left in FIG. 1) it is possible, by firing all nozzles throughout the pass, to print the next 16 pixel rows. By repeating the process for successive passes of the print head, larger and larger areas of the paper may be "blocked in." This is especially important when printing graphics having solid regions of significant size.
FIG. 2 shows the nozzles fired during the first four passes of the print head and the pixel rows which each nozzle fills. In a first pass (left to right; direction R), nozzles 1-16 are fired (as indicated by the filled-in dot adjacent each nozzle number) onto pixel rows 1-16. The paper is then stepped so as to position pixel row 17 opposite nozzle 1. Then, in a second pass (right to left; direction L), nozzles 1-16 are fired onto pixel rows 17-32. The paper is then stepped so as to position pixel row 33 opposite nozzle 1. Then, in a third pass (left to right; direction R), nozzles 1-16 are fired onto pixel rows 33-48. The paper is then stepped so as to position pixel row 49 opposite nozzle 1. Then, in a fourth pass (right to left; direction L), nozzles 1-16 are fired onto pixel rows 49-64.